So I bought this and when I landed in the States, got a T mobile Pay as you go sim (unlimited calls to the UK for $50 a month, no contract!!!).
Initially, when I would switch from UK to US, it wouldn't lock on to the US network at all. Very tempramental.
However, after hours of tinkering (I REALLY didn't want to carry two phones around and the iPhone has great stuff like Google Maps which has been a godsend), I found that if you switch to the US sim, go to settings---carrier and switch off automatic and manually select your carrier and leave it like that. You may have to scan for it to pick up the network you require maybe 5 or even 10 times. Like I said, it's tempramental, and the same process whenever you switch back to the UK sim and to the US again.